Some collectable blind box series have even odds, like many of the recent Mystery Mini series from Funko which are 1/12. This means that if you got a display case of 12, you’d get one of each one. Vinylmation on the other hand sold cases of 24 that included two full sets and one “chaser”. This means that if you had a full case, you’d get two of each one (1/11) (three of one of them) plus one item that was only in the case once (1/24). Some “chasers” are even harder to get your hands on with even smaller odds. Kidrobot often has items with some very rare odds, for example, the In the Brief series of zipper pulls contains several designs that are 1/25 but some are 1/50 and two are even 1/100.
But really, a chaser isn’t necessarily rare. It usually is, but not always. In some cases you’ll find a mystery design with reasonable odds such as with Kidrobot’s Toby’s Secret Society series. It’s mystery figure is 1/16 like many other designs in the series. But normally, you’ll find the chaser is both a hidden design (typically a silhouette of the shape with no detail) and among the most rare of the designs. A good example of this is Kidrobot’s Fatcap series 2 which has a chaser that is both a mystery design and very rare at just 1/200. Another in this same series offers us an example of mystery odds, where you are not told how common or rare a figure is– identified as “?/??” (often referred to as mystery odds).
The Be@rbrick series contains a number of rare “chase” figures as well as many rare designs called right out on the box, but they list odds as percentages instead of the traditional fraction method.